The catalytic fluorination of halogenated hydrocarbons in the gas phase by HF is a well-known method of access to fluorinated hydrocarbons and in particular to hydrofluorocarbons (HFC). The latter are currently used as substitutes for chlorofluorocarbons (CFC), which are suspected of contributing to the weakening of the stratospheric ozone layer.
French Patent Application FR 2,669,022 thus makes known a mixed catalyst composed of nickel and chromium oxides, halides and/or oxyhalides deposited on a support composed of aluminium fluoride or of a mixture of aluminium fluoride and alumina.
The content by weight of nickel and chromium mentioned for the said catalyst is between 0.5 and 20% for each metal, the nickel/chromium atomic ratio being between 0.5 and 5. This catalyst is used for the preparation of 1,1,1,2-tetrafluoroethane (also known as F134a) from 1-chloro-2,2,2-trifluoroethane (F133a). F134a is an HFC, used in particular for refrigeration and freezing, which has no harmful effect on the stratospheric ozone layer.
Application FR 2,669,022 mentions maintenance of the effectiveness over time of the catalyst for a duration ranging up to approximately 400 hours of operation.
However, it is highly desirable to have available a catalyst which, under the conditions for the industrial production of fluorinated hydrocarbons, retains its activity over an operating period of several thousand hours.
The increase in the lifetime of the catalyst for such operating periods is all the more desirable since the decrease in activity, when it occurs, cannot be put right by an in situ regeneration treatment but then requires the shutdown of the production unit in order to replace the charge of the catalyst.
A catalyst which makes it possible to decrease the number of shutdowns of the production unit and which makes it possible, for a given charge of the said catalyst, to produce a greater amount of desired final product is particularly advantageous with respect to the running of the production unit and the cost price of the desired final product.